In the field of printed wiring board manufacturing there are commonly employed photosensitive elements (laminated bodies) having a structure wherein a photosensitive resin composition or a layer comprising the photosensitive resin composition (hereinafter referred to as “photosensitive resin composition layer”) is formed on a support film, as a resist material for etching or plating, and a protective film is situated on the photosensitive resin composition layer.
Printed wiring boards have conventionally been produced, using such photosensitive elements, by the following procedure. Specifically, the photosensitive resin composition layer of the photosensitive element is first laminated onto a circuit-forming board such as a copper-clad laminate. The side of the photosensitive resin composition layer (hereinafter referred to as the “upper side” of the photosensitive resin composition layer) opposite from the side in contact with the support film (hereinafter referred to as the “lower side” of the photosensitive resin composition layer), is contact bonded to the side of the circuit-forming board on which the circuit is to be formed. Therefore when the protective film is positioned on the upper side of the photosensitive resin composition layer, the lamination operation is carried out while peeling off the protective film. The lamination is accomplished by thermocompression bonding the photosensitive resin composition layer to a ground layer circuit-forming board (ordinary pressure lamination).
The photosensitive resin composition layer is then subjected to pattern exposure through a mask film or the like. The support film is released at a timing of either before or after the exposure. The unexposed sections of the photosensitive resin composition layer are removed by dissolution or dispersion with a developing solution. Etching treatment or plating treatment is then performed to form a pattern, and finally the cured sections are peeled off.
The etching treatment referred to here is a method for etching removal of the metal surface of a circuit-forming board that is not covered with the cured resist formed after development, and then releasing the cured resist. Plating treatment, on the other hand, is a method that involves plating treatment with copper or solder on the metal surface of a circuit-forming board that is not covered with the cured resist formed after development, removing the cured resist and then etching the metal surface that has been covered by the resist.
Incidentally, the method for the pattern exposure may be a conventional method for using a mercury lamp as the light source for exposure through a photomask. In recent years, direct writing exposure has been proposed as a new exposure technology of DLP (Digital Light Processing), wherein the digital data of a pattern is directly drawn onto a photosensitive resin composition layer. Direct writing exposure has more satisfactory positioning precision than exposure through photomasks, while fine patterns can also be obtained, and it is therefore being introduced for formation of high-density package boards.
In pattern exposure, the exposure time must be shortened as much as possible to improve the production throughput. In the direct writing exposure mentioned above, a long exposure time will generally be necessary when using a composition with sensitivity comparable to that of a photosensitive resin composition used for conventional exposure through a photomask. It is therefore necessary to increase the illuminance on the exposure device side or increase the sensitivity of the photosensitive resin composition.
It is important for the photosensitive resin composition to have excellent resolution and adhesiveness, as well as an excellent resist release property, in addition to sensitivity. If the photosensitive resin composition can provide a resist pattern with excellent resolution and adhesiveness, it will be possible to sufficiently reduce short circuits and wire breaks.
If the photosensitive resin composition is able to form a resist with an excellent release property, the shorter resist release time will improve the resist pattern forming efficiency, and a smaller size of the resist release sections will reduce resist release residue and improve the yield of circuit formation.
In a high-density package board, it is also important for the resist shape to be excellent because of the narrow widths between circuits. If the cross-sectional shape of the resist is trapezoidal or reverse trapezoidal, or the resist has skirt trailing, the circuit formed by the subsequent etching treatment or plating treatment may exhibit shorting or wire breakage. The resist shape is therefore preferably rectangular.
Photosensitive resin compositions employing specific binder polymers, photopolymerization initiators or the like have been studied to meet this need (see Patent documents 1 and 2, for example).